The present invention relates to optical fibers and, more particularly, to providing dispersion compensation in an optical fiber transmission system over a broad range of wavelengths.
Dispersion in a glass fiber causes pulse spreading for pulses that include a range of wavelengths, due to the fact that the speed of light in a glass fiber is a function of the transmission wavelength of the light. Pulse broadening is a function of the fiber dispersion, the fiber length and the spectral width of the light source. Dispersion for individual fibers is generally illustrated using a graph having dispersion on the vertical axis (in units of picoseconds (ps) per nanometer (nm), or ps/nm) or ps/nm-km (kilometer) and wavelength on the horizontal axis. There can be both positive and negative dispersion, so the vertical axis may range from, for example, xe2x88x92250 to +250 ps.
For example, typical single mode fibers generally transmit best (i.e., with minimum attenuation) at 1550 nm, whereas dispersion for the same fiber would be approximately zero at 1310 nm. The theoretical minimum loss for glass fiber is approximately 0.16 db/km, and that occurs at the transmission wavelength of about 1550 nm. Because minimum attenuation is prioritized over zero dispersion, the wavelength normally used to transmit over such fibers is typically 1550 nm. Also, Erbium-doped amplifiers, which currently are the most commonly used optical amplifiers for amplifying optical signals carried on a fiber, operate in 1530 to 1565 nm range. Because dispersion for such a fiber normally will not be zero at a transmission wavelength of 1550 nm, attempts are constantly being made to improve dispersion compensation over the transmission path in order to provide best overall system performance (i.e., low optical loss and low dispersion).
Many techniques have been used for dispersion compensation, including the design and use of dispersion-shifted and dispersion flattened fibers. Dispersion Compensating Modules (DCMs) have also been used in optical communications systems for dispersion compensation, especially in wavelength division multiplexing (WDM) systems. A number of patents describe various uses of DCMs to compensate dispersion including: U.S. Pat. No. 4,261,639 (Kogelnik et al.); U.S. Pat. No. 4,969,710 (Tick et al.); U.S. Pat. No. 5,191,631 (Rosenberg); and U.S. Pat. No. 5,430,822 (Shigematsu et al.). These patents compensate dispersion by inserting DCMs at appropriate intervals along the transmission path. The DCMs usually contain Dispersion Compensating Fiber (DCF) of an appropriate length to produce dispersion of approximate equal magnitude (but opposite sign) to that of the transmission fiber.
One problem with using the known DCMs to compensate dispersion is that DCF designs generally are sensitive to production tolerances. Therefore, if the DCF design is not highly precise, then when the DCF is combined with the transmission fiber, the resulting transmission link may have too much residual dispersion (i.e., dispersion on wavelength channels other than the center wavelength channel being compensated). This is especially true in broadband communications systems in which transmission rates are very high (e.g., 40 gigabits per second (Gbit/s)). Also, once the DCF is produced, only the length of the DCF can be selected to meet the desired target for dispersion compensation. Moreover, selection of the DCF length (and thus the dispersion of the DCM) should ensure that first order and higher order dispersion are compensated.
When compensating for higher order dispersion, the Relative Dispersion Slope (RDS) of the transmission fiber should match the RDS of the DCF (and, consequently, of the corresponding DCM). For a given fiber, the RDS is defined as the ratio of the dispersion slope, S, of the fiber to the dispersion, D, of the fiber. Thus, the RDS for a given fiber is equal to S/D for that fiber. For a DCF combined with a transmission fiber, the total dispersion and the total dispersion slope of the compensated link, DLINK and SLINK, respectively, can be expressed by Equations 1 and 2, respectively, as follows:
DLink=DTransmFiberxc3x97LTransmFiber+DDCFxc3x97LDCFxe2x80x83xe2x80x83(Eq. 1)
SLink=STransmFiberxc3x97LTransmFiber+SDCFxc3x97LDCFxe2x80x83xe2x80x83(Eq. 2)
In Equation 1, DTransmFiber corresponds to the dispersion of the transmission fiber, LDCF corresponds to the length of the DCF, and DDCF corresponds to the dispersion of the DCF. In Equations 1 and 2, LTransmFiber corresponds to the length of the transmission fiber and LDCF corresponds to the length of the DCF. In Equation 2, STransmFiber corresponds to the dispersion slope of the transmission fiber and SDCF corresponds to the dispersion slope of the DCF.
When the dispersion of the system is compensated, i.e., when DLink=0, the length of DCF needed to compensate for the dispersion of the link can be determined by Equation 3 as follows:
LDCF=LTransnFiber/DDCF)xc3x97LTransnFiber.xe2x80x83xe2x80x83(Eq. 3)
In order to compensate the link for the dispersion of the DCF, the RDS for the DCF and for the transmission fiber are matched such that:                               RDS          TramsFiber                =                                            S              TramsFiber                                      D              TramsFiber                                =                                                    S                DCF                                            D                DCF                                      =                          RDS              DCF                                                          (                  Eq          .                      xe2x80x83                    ⁢          4                )            
It is desirable that the compensated wavelength range be as wide as possible. An inverse relationship exists between the usable bandwidth and the RDS of the transmission fiber. Thus, the RDS of the transmission fiber limits the usable bandwidth of the transmission link.
It would be desirable to provide a dispersion slope compensation module that increases the usable bandwidth of a dispersion compensated transmission fiber, lo compared to the usable bandwidth associated with using a single dispersion compensating fiber, especially in cases where the transmission fiber has a high RDS.
In accordance with the present invention, it has been determined that the usable bandwidth of an optical fiber transmission link can be increased by coupling the transmission fiber of the link with a positive dispersion compensation fiber (hereinafter xe2x80x9cDCF+xe2x80x9d) that has a positive dispersion and a dispersion slope such that the relative dispersion slope (RDS) of the DCF+ is lower than the RDS of the transmission fiber. Because the RDS of the combination of fibers is lower than the RDS of the transmission fiber, when the transmission link is compensated, there is an increase in the usable bandwidth of the transmission link.
The present invention provides a dispersion compensation module (DCM) for increasing the usable bandwidth of the optical fiber transmission link after dispersion compensation with negative dispersion compensating fiber. The DCM comprises at least one DCF+ that has an RDS that is lower than the RDS of the transmission fiber. When the DCM is coupled to the transmission fiber, the DCF+ adds an amount of positive dispersion. Because the RDS of the transmission fiber is higher than that of the DCF+, the overall RDS of the transmission link is lowered below that of the transmission fiber, which results in more usable bandwidth for the transmission link when it is compensated.
The present invention also provides a DCM that contains both a DCF+ as described above and negative dispersion compensation fiber (hereinafter a xe2x80x9cDCFxe2x88x92xe2x80x9d) with a dispersion of equal amount and opposite sign to that of the combined transmission fiber and DCF+. The DCFxe2x88x92 has an RDS that is at least substantially equal to that of the combined transmission fiber and DCF+.